The present invention is directed to a loudspeaker system, and in particular to using differential flow vents within a loudspeaker enclosure and within the backplate of a loudspeaker magnet structure to provide preferential airflow to cool and ventilate the system and to compensate for non-linear effects created by dynamic loudspeaker offset such as that found in a woofer.
Reference is made to FIG. 1 which illustrates a conventional loudspeaker 10 using a cone-shaped diaphragm 12 and a voice coil 14 formed by having wire wound around an annular coil form 16 made from a heat resistant material which may be metallic or plastic-based. The diaphragm 12, otherwise known as a speaker cone, vibrates through an electro-mechanical drive. The wire used in forming the voice coil has a coating for insulation purposes. The material of such coating may include shellac, an epoxy material, or varnish, wherein the coil windings are cemented onto the coil form. The voice coil defines a voice coil chamber 18 therein. The coil form is attached to a spider element 20. The spider element is fabricated from a resin impregnated clothlike material and has circular corrugations formed therein. The spider element resiliently supports the coil form from a loudspeaker frame, also known as a basket 22 which typically comprises a metal material. In certain situations, the spider may be integrally formed with the coil form. The basket may be attached to the speaker enclosure by, a gasket 24. The cone 12 may be fabricated from well-known materials and is attached to the coil form 16 at one end, while attached to a speaker surround 26 at the other, possibly with the use of gasket 24. The surround is attached to the basket. The voice coil operates in a conventional manner in an annular gap 28, which is positioned between a center pole piece 30 and an annular magnet 32. The pole piece and magnet causes the mechanical actuation of the voice coil and the coil form about the voice coil chamber in response to electrical signals received at the coil. This in turn causes the speaker cone diaphragm to vibrate with acoustical energy. A backplate 34 and top plate 36 help secure the above mentioned components in place and direct the magnetic field from the magnet. A protective dust cap 38 is placed over the voice coil chamber.
When the electrical signal or current is supplied to the voice coil, the speaker cone vibrates in accordance with the audio frequency and polarity of the electrical signal. The winding used to form the coil has an electrical resistance to the flow of current and generates heat. This heat increases the temperature within the loudspeaker and the corresponding enclosure. As heat is generated in the voice coil, it is conducted away from the coil by means of both the thermally conductive voice coil form and the front plate. These function to dissipate the heat energy. Thus, a portion of electrical power input towards driving the speaker is converted into heat as opposed to acoustic energy. For high power loudspeaker systems, the temperature of the voice coil and the loudspeaker enclosure correspondingly increases. Accordingly, the components used in the loudspeaker control the ability of the loudspeaker to tolerate heat. When the capacity of heat dissipation of the loudspeaker components is exceeded, overheating occurs.
To prevent overheating and to provide loudspeaker cooling, methods to remove heat energy have been suggested since the operation and performance of the loudspeaker is directly affected by the heat tolerance level. For example, it is well known to cool a loudspeaker by using a heat sink. U.S. Pat. No. 4,138,593 to Hasselbach et al. discloses an extended heatsink in contact with the loudspeaker magnet structure for dissipating heat across the enclosure housing. However, the hot air remains in the interior of the enclosure. U.S. Pat. No. 4,210,778 to Sakurai et al. discloses a heat pipe device for transferring heat over a distance to be vented out of the cabinet. The heat pipe is attached to the magnet structure and draws heat from the magnet structure, and the terminus of the heat pipe is centered within the vent of the enclosure. Yet, this appears to be unsatisfactory because any airflow pertaining to the action of the vent is not a continuous motion, but encompasses air in the vent which is merely oscillating back and forth with no net travel. What is needed is a manner of exhausting the heated air out of the enclosure.
The prior art attempts to exhaust this heated air through the use of vents positioned within the loudspeaker enclosure. U.S. Pat. No. 4,196,792 to Grieves et al. discloses a V-slot or V-shaped vent installed in the back wall of the enclosure of a speaker assembly and suggests that the vent prevents pressure build-up and whistling. U.S. Pat. No. 4,284,166 to Gale discloses a port opening in a speaker enclosure for free movement of air outwardly and inwardly. U.S. Pat. No. 3,778,551 to Grodinsky discloses holes in the speaker cabinet which open to the outside and lead via an air passage to the power transistors. Using the speaker cone as an air pump, air is forced into and out of the cabinet for cooling the transistors. Japanese Patent Application No. 6-141396 discloses a series of passages through the front plate of a speaker to allow air circulation. U.S. Pat. No. 5,533,132 to Button discloses air movement within the loudspeaker to aid in heat dissipation, and in particular, the embodiment comprises a symmetrical pair of air vents in the enclosure in which the air moves in and out at high velocity so as to act as a fan on a vaned heat sink. But with all of these techniques, the problem with providing simple holes or vents in a speaker enclosure is that air only moves back and forth in the vent, even with the use of the speaker cone as an air pump. Moreover, small openings or holes do not act preferentially by themselves and tend to be acoustically resistive, constituting an acoustical leak which lowers the quality factor or efficiency of the enclosure.
The difficulty with using these vents of the prior art may be better recognized through an understanding of the hole or vent being idealized. In an idealized or theoretically perfect vent, the same slug of air moves back and forth within the vent. The more this slug of air is disturbed or broken up, the less efficient the vent acts as an acoustical element. If the slug of air is never changed, it would heat up and reach thermal equilibrium with the surroundings. Due to turbulence and any net air motion outside of the enclosure, a portion of the slug of air will be very slowly exchanged over a period of time for different xe2x80x9cfreshxe2x80x9d air. As a result, the slug of air will continue to heat up and reach thermal equilibrium with the surroundings. The net effect is only a small amount of cooling.
Likewise, in another attempt to provide loudspeaker cooling, U.S. Pat. No. 4,928,788 to Erickson discloses a ported reflex speaker enclosure and a method which is somewhat similar to convection cooling techniques that capitalizes on the natural tendency of heated air to rise. Although it is suggested that heated air is exhausted from the enclosure via the port, under principles of convection cooling, hot air tends to rise and thus, any air flowing from the hole residing on the bottom of the speaker housing would consequently not aid in the cooling of the speaker. Additionally, notches or openings near mounting holes in the speaker system are discussed to complement cooling; however, they appear to function as resistive vents, or some sort of pressure relief and with the presence of a larger bass reflex vent, these openings will be effectively short-circuited acoustically and see little or no pressure. Accordingly, what is needed is an improvement over the conventional vent of the prior art, and in particular, a vent that provides airflow in a preferred direction. It is desirable to overcome the drawback of the conventional vent having a slug of air moving back and forth by providing the improved vent which will allow preferential airflow. Preferential airflow means that the air flows more easily in one direction (a preferential direction) than in the opposite direction. Therefore, more air will flow in said one direction than in said opposite direction as long as the operating conditions of the loudspeaker device remains the same.
Additionally, what is also needed is a manner of placing the improved vent within the speaker enclosure to ensure preferential airflow of fresh air into the enclosure and preferential airflow of heated air exhausted out of the enclosure all to accomplish significant cooling of the enclosure, and not merely the minor incidental cooling effects of the prior art. It would be ideal to use the natural motion of the speaker cone as a pump along with such an improved vent to positively circulate air throughout the loudspeaker system to cool heated components.
Other methods of cooling a loudspeaker includes a liquid cooling method, wherein Japanese Patent Application No. 3-23909 to Saito discloses inlet and outlet tubes through which an external source of cooling medium like liquid air is supplied to cool a loudspeaker assembly. However, the method has drawbacks because an external source must be turned to, thus making the fabrication and manufacturing process more complex and costly. Furthermore, other methods involve mechanically forcing air through vents. For example, U.S. Pat. No. 3,991,286 to Henrickson discloses a blower used for circulating air to the heat sink. Yet, such air circulation must be powered by an external device, making this apparatus expensive to manufacture. U.S. Pat. No. 4,811,403 to Hendericksen et al. also utilizes an externally powered fan to force air to cool the loudspeaker through two openings or aperatures, but, this air flow is controlled by the fan. Also, the incorporation of an airflow channel acts as an acoustical leak and significantly reduces the efficiency of the vented enclosure. U.S. Pat. No. 4,757,547 to Danley discloses a blower or fan used to cool the magnet structure through air passages denoted as inlets and outlets, yet, the fan is powered by a signal robbed directly from the speaker and forces movement of air throughout the loudspeaker. This is disadvantageous because using the same signal that is sent to the speaker to also power the fan creates distortion due to the loading of the full wave bridge rectifier. Moreover, this lowers efficiency because tapping from this signal also steals power that would otherwise be used to make the loudspeaker play louder. It is thus desirable to provide loudspeaker cooling using the above-mentioned improved vents placed in speaker enclosure in a manner that provides positive airflow circulation, and yet in a manner which eliminates reliance upon an external device requiring power. It would be cost-effective if these improved vents are used with the natural motion of the speaker cone acting as a pump.
While cooling the loudspeaker is of primary concern with such an improved vent, it is also desirable to provide an improved vent that helps but never hinders the acoustics of the loudspeaker system. Referring for illustrative purposes to a particular type of loudspeaker such as a woofer providing lower acoustical sound frequencies, in a vented bass reflex cabinet, the woofer cone motion alternately pressurizes and depressurizes the interior of the cabinet enclosure, forcing air in the vents or ports to move back and forth. At the resonant frequency of the vented cabinet system, the woofer cone motion is all but nil, while the air velocity in the vents is high. The cabinet air volume comprises an acoustical analog to a capacitor. The mass of air in the vent or port comprises an acoustical analog to an inductor. In order for the vented bass reflex resonant system to maximize bass output in a desired manner, the nature of these resonant elements should be as perfect and as pure as possible. That is, the air inside the cabinet should not be subject to any leaks, other than the vents themselves, and the walls of the cabinet should be perfectly stiff. This allows maximization of the purity of the capacitance represented by the enclosed air volume. In the same way, the mass of air moving back and forth in the vent should maintain its integrity as much as possible. A perfect vent would always have the same air moving back and forth within it. Often times however, the woofer has an inherent dynamic offset and tends to move either forward or backward, thereby posing an additional parameter for causing distortion and system non-linearity.
The Thiel/Small theory of vented cabinet design calls for a vent cross-sectional area equal to the area of the woofer cone radiating area. Practical real world-constraints typically do not allow for a vent this large, for the larger the area, the longer a vent has to be for a given tuning frequency. The practical rule of thumb to prevent significant reduction in vent efficiency, requires making the vent area approximately xc2xc the woofer cone radiating area. As the area of the vent decreases, the velocity in the vent increases as do the losses. At some point, the vent will begin to make spurious noise and whistle. Inefficiencies arise when the air in the vent becomes disturbed, due to turbulence from excessive velocity. This occurs in a vent that is too small. Other factors that causes the air in the vent to become disturbed include rough edges, the cabinet having less than perfectly rigid walls and air leaks. As such, it would be desirable to provide the improved vent mentioned above in a manner that retains vent efficiency. It would be ideal if the improved vent were able to compensate or correct non-linearities, such as the distortion caused by a woofer offset or by less than perfect loudspeakers with less than optimally sized vents. What is ultimately needed is a manner of providing such an improved vent that accounts for the aspect of balancing the vent actions to provide symmetry and linearity so as to improve acoustics. Thus far, the prior art fails to mention the use of a differential flow vent for correcting system non-linearities.
Besides placing vents within the loudspeaker enclosure and forcing air into the enclosure to cool the loudspeaker system, the prior art also suggests directly cooling the electro-mechanical driving components supported within the enclosure. U.S. Pat. No. 4,625,328 to Freadman discloses a heat sink attached to the magnetic structure""s front plate, and the motion of the cone is supposedly causing air waves to increase circulation in the area of the heat sink. The motion of the air, however, flows back and forth and any air exchange to provide long term cooling is incidental. It should also be noted that the air immediately behind the speaker cone is usually within an enclosure, and thus there is little opportunity for large scale air exchange, even with incidental local turbulence. French Patent Application No. 2,667,212 to Maurice discloses fins just below the spider for moving air back and forth; however, a similar problem exists in that the air is merely moved as opposed to being pumped in a preferential direction.
U.S. Pat. No. 5,042,072 to Button discloses cooling the voice coil directly by forcing air displaced by movement of a dome-shaped diaphragm through channels next to the voice coil to and through vents located in the magnet structure of the voice coil. However in practice, this system has drawbacks because air is merely moved back and forth, oscillating in place, and there is no net air movement provided. When such a speaker is placed in a typical enclosure, the air trapped inside will not allow even the incidental turbulence to provide very much long term cooling for the loudspeaker""s magnet structure. Furthermore, these vents require modifications from a typical structure to the front plate and the pole piece. This reduces the amount of magnetic flux due to the removal of significant portions of the front plate in proximity to the voice coil.
U.S. Pat. No. 5,246,353 to Sohn discloses gaskets used on the voice coil assembly and a bellows assembly to pump air. Inlet and outlet vents, however, are located on the vibrating medium and it is suggested that air breezing is used to provide an air pressure bias and to maintain pressure on a thin and normal flexible speaker diaphragm. However, the intake of air will always be in equilibrium with hot air being exhausted from the enclosure. As such, air is not positively circulated about the enclosed volume. Similarly, U.S. Pat. No. 5,497,428 to Rojas discloses air vents and two air channels used in cooling the voice coil, however, the same stale air oscillates whereby it moves back and forth, yet does not flow in a definite or preferred direction.
U.S. Pat. No. 5,357,586 to Nordschow et al. discloses multiple air flow paths used to cool the speaker, as well as cabinet vents. However, the paths in the magnet structure are too complicated to manufacture cost-effectively. Moreover, issues of system linearity and acoustical balancing of vents are not addressed. While it is suggested that vent efficiency is improved with cabinet cooling, placing added member 40 in the vent tube reduces the vent area severely to perhaps ⅕ of its original area. This is contrary to known acoustic principles, where vent efficiency is measured by how close the vent action approximates the theoretical ideal. Vent area relates directly to vent efficiency; and thus, by placing added member 40 in the vent tube, vent tuning would change appreciably and either necessitate a greater number of vents or that the vent be scaled up.
As there have been air-cooled speaker cabinets with external fans, there too have been air-cooled speaker magnet structures facilitated by an external power source, whether that source is a 120 VAC or compressed air. Other approaches place a full wave bridge rectifier through a resistor across the speaker inputs, and convert some of the AC input signal into filtered DC to run a fan. Yet, these approaches generate distortion due to the loading of the bridge rectifier and the filter caps, in addition to robbing power to the woofer.
Consequently, it is desirable to directly cool a loudspeaker, such as a woofer magnet structure, using an improved vent that provides preferential airflow in a single direction and that ensures a net air exchange that is increased substantially beyond the incidental and minimal exchange seen in the prior art. Like the need for cooling the speaker enclosure, what is needed is an improved vent that would directly cool the heated electro-mechanical components with positive air circulation well beyond the merely moving air back and forth as in the prior art. It would be advantageous if the improved vent were able to compensate or correct less than perfect loudspeakers and eliminated non-linearities.
The present invention provides differential flow vents for controlling airflow to be in a preferred direction, not merely back and forth as in the conventional vents of the prior art. A loudspeaker device with differential flow vent means has an enclosure being substantially closed to define an interior space. A loudspeaker assembly is supported within this space, and the differential flow vent means provides communication between the space and ambient air. The flow vent means define a first cross-sectional area and a second cross-sectional area smaller than the first cross-sectional area and produce a greater resistance to airflow in one direction from the second cross-sectional area to the first cross-sectional area than in the opposite direction. By deliberately constructing differential flow vents to be non-linear, the airflow through the vent is preferential.
The differential flow vent means may be constructed to be a tapered portion having a first end and an opposite second end. The first end defines the first cross-sectional area and the second end defines the second cross-sectional area. The differential flow vent means is disposed within the enclosure and has an overall length, while the tapered portion has a length which is substantially less than the overall length. In one embodiment, the tapered portion is of generally frusto-conical configuration and may be joined with a cylindrical portion. In another embodiment, the differential flow vent means includes a first pair of generally parallel spaced walls and a second pair of generally parallel spaced walls extending substantially perpendicular to the first pair of walls. In this embodiment, the differential flow vent means comprises a plurality of flanges defining a first end and a second opposite end, wherein the flanges comprises a first pair of flanges tapering toward one another from the first pair of walls and a second pair of flanges spaced from the first pair of flanges and tapering toward one another from the second pair of walls. In other embodiments, the first pair of flanges may be used and alternatively even one flange by itself.
By placing differential flow vents within a loudspeaker cabinet enclosure and by using the natural motion of the speaker cone, fresh air may be preferentially pumped into the enclosure and heated air may be preferentially pumped out of the enclosure. With this implementation, positive air circulation is provided to cool the cabinet enclosure without directly having to cool the loudspeaker magnet structure.
In order to directly cool the loudspeaker magnet structure, such as a woofer assembly, differential flow vents are placed within the backplate of the woofer assembly so that cabinet air is preferentially pumped into and out of the woofer assembly using the natural motion of the cone. According to this implementation, positive air circulation is provided to thereby directly cool the voice coil and electro-mechanical components that have become heated.
By placing differential flow vents in both the loudspeaker enclosure and backplate of the woofer assembly, maximal cooling of the loudspeaker system is achieved.
But besides cooling the loudspeaker system, it is an object of this invention to use differential flow vents in a manner that compensates for non-linearities created by the loudspeaker offset. In one embodiment, a single differential flow vent is placed within the cabinet enclosure to compensate or correct the inherent forward or backward movement of a woofer. In a preferred embodiment, a pair of opposed mildly asymmetrical differential flow vents are placed in the cabinet enclosure to pressurize or depressurize the cabinet enclosure with the natural motion of the speaker cone acting as a pump. One differential flow vent acts as an intake vent and the other as an out-take vent. Effectively, one vent provides greater resistance to airflow from ambient air into the cabinet space, while the other vent provides greater resistance to airflow from the space to ambient air. Between the pair of vents, the resistance to airflow of one vent is greater than that of the other. This respectively cancels the forward or backward inherent movement of the woofer and provides air circulation in a definite direction which cools the enclosed cabinet supporting electronics for a powered speaker system, or for the woofer along with an associated magnet structure when handling a large amount of input power. Moreover, whereas the conventional vents used in the prior art created additional distortion for lack of opposing or balancing elements, these distortions are minimized with the current differential flow vents.
In yet another embodiment, the same result of cooling the loudspeaker while compensating for non-linearities is accomplished. Instead of a single pair of opposed asymmetrical differential flow vents, multiple pairs are used to pressurize or depressurize the cabinet depending upon the severity of the inherent forward or backward movement of the woofer assembly. Alternately in still another embodiment, an odd number of opposed asymmetrical differential flow vents are used in the cabinet enclosure to cool the cabinet and to compensate for non-linearities. The number of vents is varied based upon the degree of compensation required to correct the woofer offset. Of course, where it is desired to not affect the symmetry of the woofer, opposed symmetrical vents are used.
When cooling the loudspeaker magnet structure directly, pairs of opposed symmetrical vents are placed in the woofer backplate when it is desired to not affect the symmetry of the woofer. But, when the inherent offset of the woofer must be compensated for, asymmetrical pairs are used. In a preferred embodiment, differential flow vents in the magnet structure are used with a foam resistance plug in place of a center pole piece for compensating for non-linearities.
With the current invention, the loudspeaker cabinet and magnet structure are cooled without dependency upon any external sources of energy or power other than the normal signal applied to the loudspeaker to move it and to make sound. Unlike the prior art, the present invention does not rob power directly from the loudspeaker, but instead utilizes the relationship between the natural movement of the speaker cone, the speaker enclosure and the differential flow vents in a cost-effective manner to create preferential air flow through the enclosure.
Furthermore, the current invention uses defined and purposeful non-linearity to do desired work, yet without adversely affecting normal vent operations. In particular, the differential flow vents are placed within the enclosure to prevent and to minimize extraneous noises and distortion, such as whistling or spurious tones caused by complex flow patterns when cooling a woofer. This invention also overcomes the modifications suggested in the prior art to the woofer structure, speaker frame, and pole pieces in an attempt to minimize extraneous noises. Additionally, this invention avoids the problems of the prior art where the amount of attempted air movement is so great and involved that it is possible that the woofer would be rendered unfit for normal use.
Still further, the present invention provides an implementation which is much simpler and less costly to manufacture. A backplate of a woofer assembly manufactured with differential flow vents is much simpler and cheaper to manufacture than the structures suggested by the prior art because to directly cool the woofer assembly merely requires drilling or incorporating straight holes through the rear plate or possibly the front plate and constructing the differential flow vents by inserting a simple small molded insert as the flange piece. Any flanges used in this invention are substantially less involved than the molded piece of the prior art, and alternately, they may be extruded for manufacturing economy. These methods are applicable not only to placing vents in the woofer backplate structure, but to the loudspeaker enclosure also. No other modifications to the standard loud speaker and magnetic structure are required, as opposed to complex custom die-casting of the frame, pole piece and front plate as seen in the prior art.
Lastly, the present invention accommodates smaller speakers like midrange speakers and large format compression tweeters, whereas with the prior art, due to size and complexity, changes to a typical speaker structure are constrained by any remaining space available.
Other objects and advantages will be apparent from the specification and drawings which follow.